Summary:
Ocean circulation drove an unexpected rise in salinity between about 40°S and 50°S from 2004 to 2024, particularly in the Pacific and Atlantic sectors of the Southern Ocean, despite expectations that an intensifying water cycle should make these already low-salinity waters even less saline.
Researchers from Woods Hole Oceanographic Institution (WHOI) traced the change mainly to the poleward expansion of Southern Hemisphere subtropical gyres. As these large wind-driven circulation systems shifted southward, they transported saltier subtropical water into higher latitudes. Across the 40–50°S band, sea surface salinity increased by about 0.03 per decade, with the most coherent changes occurring in the Pacific and Atlantic sectors.
The analysis, published in Nature Communications, shows that horizontal ocean transport was the dominant driver, accounting for roughly three times the domain-mean contribution of surface freshwater flux, which acted in the opposite direction. The salinity increase also extended below the surface to depths of about 500 metres in the Pacific and Atlantic.
The study suggests that shifts in ocean circulation can counteract freshwater-driven changes, with potential implications for Southern Ocean density, stratification, ventilation, nutrient cycling and the Southern Ocean’s role in regulating heat and carbon.
Read also: Ocean eddies reshape coastal seas as warming accelerates at the surface
— Press Release —
Subtropical gyre expansion causes Southern Ocean salinification contrary to freshening predictions
The “salty-gets-saltier, fresh-gets-fresher” theory predicts that an intensifying water cycle should freshen the climatologically fresh Southern Ocean. In a new study, scientists show the opposite: sea surface salinity increased across latitudes 40–50°S during 2004–2024, mostly in the Pacific and Atlantic sectors. They attribute this salinification to the poleward expansion of the southern subtropical gyres, which transports salty subtropical water into latitudes of steepest meridional salinity gradient.
A gyre is a large system of rotating ocean currents. There are five major gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre. This study focused on the South Pacific, South Atlantic, and Indian subtropical gyres which dominate midlatitude circulation in the Southern Hemisphere. The individual gyre in each basin is characterized by persistent counterclockwise surface flow driven by the wind stress.
Within the core of these gyres, more evaporation, less rain, and Ekman convergence (where wind pushes water movements together, forcing surface water to sink) lead to increased sea surface salinity, making these among the saltiest surface waters on Earth.
Surface currents are influenced by the wind. However, other, much slower currents that occur from the surface to the seafloor are driven by changes in the saltiness and ocean temperature, a process called thermohaline circulation. These currents are carried in a large global conveyor belt. This conveyor belt includes the Atlantic Meridional Overturning Circulation (AMOC).
The circulation process begins as warm water near the surface moves toward the poles (such as the Gulf Stream in the North Atlantic), where it cools and forms sea ice. As this ice forms, salt is left behind in the ocean water. Due to the large amount of salt in the water, it becomes denser, sinks down, and is carried southwards in the depths below. Eventually, the water gets pulled back up towards the surface and warms up in a process called upwelling, completing the cycle.
The entire circulation cycle of the AMOC, and the global conveyor belt, is quite slow. It takes an estimated 1,000 years for a parcel (any given cubic meter) of water to complete its journey along the belt.
One of the broad implications found in this study was that salinification near 40–50°S due to migrating gyres may locally modulate surface density in a warming Southern Ocean, partly offsetting buoyancy gains from surface warming and freshwater input, though the net effect on stratification is likely to vary seasonally and regionally.
Journal Reference:
Yu, L., Toole, J.M., ‘Subtropical gyre expansion causes Southern Ocean salinification contrary to freshening predictions’, Nature Communications 17, 8924 (2026). DOI: 10.1038/s41467-026-75775-2
Article Source:
Press Release/Material by NOAA Research
Featured image credit: Jakob Owens | Unsplash
